Re: [PATCH] blk-mq: fix kernel panic during iterating over flush request

[Ming Lei <ming.lei@redhat.com>]

On Wed, Aug 11, 2021 at 10:26:24PM +0800, Ming Lei wrote:
> For fixing use-after-free during iterating over requests, we grabbed
> request's refcount before calling ->fn in commit 2e315dc07df0 ("blk-mq:
> grab rq->refcount before calling ->fn in blk_mq_tagset_busy_iter").
> Turns out this way may cause kernel panic when iterating over one flush
> request:
> 
> 1) old flush request's tag is just released, and this tag is reused by
> one new request, but ->rqs[] isn't updated yet
> 
> 2) the flush request can be re-used for submitting one new flush command,
> so blk_rq_init() is called at the same time
> 
> 3) meantime blk_mq_queue_tag_busy_iter() is called, and old flush request
> is retrieved from ->rqs[tag]; when blk_mq_put_rq_ref() is called,
> flush_rq->end_io may not be updated yet, so NULL pointer dereference
> is triggered in blk_mq_put_rq_ref().
> 
> Fix the issue by calling refcount_set(&flush_rq->ref, 1) after
> flush_rq->end_io is set. So far the only other caller of blk_rq_init() is
> scsi_ioctl_reset() in which the request doesn't enter block IO stack and
> the request reference count isn't used, so the change is safe.
> 
> Fixes: 2e315dc07df0 ("blk-mq: grab rq->refcount before calling ->fn in
> blk_mq_tagset_busy_iter")
> Reported-by: "Blank-Burian, Markus, Dr." <blankburian@uni-muenster.de>
> Tested-by: "Blank-Burian, Markus, Dr." <blankburian@uni-muenster.de>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>

Hello Jens,

Ping...

Thanks,
Ming